L-Ascorbic acid, also known as vitamin C, is an important dietary factor. It is not synthesized in vivo in human, monkey and guinea pig and therefore is listed as an essential nutritive element. This ubiquitous compound is essential for the maintenance and preservation of several species including human being. Scurvy has been known as a condition due to deficiency of L-ascorbic acid and is marked by weakness of the skin, petechial hemorrhage, ecchymosis, and hemorrhages in the gingiva and marrow, which can be life threatening. To prevent scurvy for the maintenance of health, a recommended daily administration is established for L-ascorbic acid; in particular, 60 mg for adult male and 50 mg for adult female.
L-ascorbic acid is probably the most effective, efficient and least toxic antioxidant. It is a water soluble, chain-breaking antioxidant. The mechanism of action involves L-ascorbic acid acting as scavenger for harmful radicals like superoxide, hydroxyl and singlet oxygen that are produced during normal or pathological cellular metabolism. Ascorbic acid is speculated to be superior to other water soluble and lipid soluble antioxidants. It also protects DNA, enzyme, protein and lipids from oxidative damage and thereby prevents aging, coronary heart diseases, cataract formation, degenerative diseases and cancer. Oxygen radicals have been implicated not only in initiation and post-initiation stages of carcinogenesis, but also in invasion and metastatic processes.
L-Ascorbic acid takes part in several important physiological functions in vivo. It is essential for collagen synthesis, proteoglygans and various components of extra cellular matrix (ECM). It helps to maintain various enzymes in their reduced forms. L-Ascorbic acid is involved in the hydroxylation of lysine and proline to the formation of hydroxylysine and hydroxproline which are necessary for the synthesis of collagen in living connective tissues. These reactions are mediated by lysly hydroxylase and prolyl hydroxylase for which L-ascorbic acid functions as cofactor. Lysine and proline are principal components of tendons, ligaments, skin, bone, teeth, cartilage, heart valves, cornea, eye lens and ground substances between cells. Hydroxproline and hydroxylysine are required for the formation of a stable ECM and cross link in the fibers. Any deficiency of L-ascorbic acid reduces the activity of prolyl hydroxylase and lysyl hydroxylase. This results in impaired formation of collagen which leads to tissue weakness and eventually, scurvy.
Medicine considers L-ascorbic acid and lysine as one of the most important natural substances that is indispensable for maintenance of health at the cellular level. Deficiency of these substances in humans may lead to various diseases. Almost all diseases are a culmination of a lack of L-ascorbic acid and lysine. Lysine inhibits the destruction of collagen molecules by binding with plasminogen and thus preventing its breakdown to plasmin, which is one of the enzymes responsible for breakdown of collagen and for activating other matrix metallo proteinases (MMPs). Since our body cannot produce its own lysine, it has to be supplied through diet or from dietary supplements. Another important amino acid, Proline, is presumed to be synthesized in the body in adequate quantities for normal functions. It is likely that under stress conditions the quantity synthesized falls short of the body needs. This often leads to a conditioned deficiency of this important amino acid.
Another hydroxproline containing protein is the plasma complement component Clq, which constitutes an integral part of the protective mechanism against pathogens. The concentration of Clq is significantly reduced in guinea pigs fed a ascorbic acid deficient diet. In addition, L-ascorbic acid participates in the biosynthesis of carnitine and neuroendocrine peptides.
L-Ascorbic acid has several reactive hydroxy groups that can be used for the synthesis of a number of derivatives. Many substituted compounds at 2-, 3-, 5- and 6-positions have been synthesized. L-ascorbate 2-sulphate is stored in fish and some shrimp. It has the L-ascorbic acid activity for fish such as trout, salmon and catfish. It is 20 times more stable than ascorbic acid. Hence, it has been used in the formulation of feeds. L-ascorbate 2-phosphate is more stable in air than ascorbic acid. This compound is used as source of L-ascorbic acid in guinea pigs and rhesus monkeys. L-ascorbyl 6-palmitate, a synthetic lipophilic ascorbic acid derivative, is another effective preservative in foods and pharmaceuticals.
In recent years there has been a growing interest in the therapeutic application of L-ascorbic acid and its derivatives as an anti-cancer agent.